Subsea fiber optical communications systems need routine monitoring to guarantee their performance and minimize potential loss of service by detecting and solving wet plant faults and possibly aggressive threats at an early stage. Currently established monitoring technologies include the use of line monitoring systems (LMS) to detect signal peaks looped back from each undersea repeater and terminal with high loss loopback (HLLB) technology. Signal peaks are expected to loopback within an expected range of time due to the fixed length of optical fiber between undersea repeaters. For example, if a repeater is 200 km away from a transmitting terminal and the group velocity of light inside the outbound optical fiber is 2×108 m/s, the signal is expected to be received back at the transmitting terminal in 2 ms within a small error based on group refractive index of the various fiber types used in the system. The error is typically small enough to be in the range of the threshold of the error of delay.
When the lengths of fibers in the system change, the expected range of time for detecting the loop-back signal peaks should be changed to avoid errors in accurate detection of undersea network elements and reporting of faults. For instance, in the previous example wherein the repeater is 200 km away and the loopback signal is expected to be received in 2 ms, it is possible that a temporary terminal configuration change increases the system length from the terminal to the first repeater by 4 km. This temporary terminal configuration change causes the original peak to be marked as undetected at 2 ms and all peaks thereafter may be interpreted as being missing (e.g. the peaks now appear to be at 204 km, 404 km and 604 km instead of 200 km, 400 km and 600 km). In effect, the LMS may interpret all repeaters as missing due to the temporary fiber length change causing the LMS to report a break between the terminal and the first repeater, when the system simply added 4 km of fiber at the terminal. A similar, but related fiber length change can occur during a repair or replacement of a repeater which typically adds two times the ocean depth of fiber at the repair site permanently.
In addition to system length changes, reflection induced anomalies can cause LMS errors. For example, non-loopback based subsea network elements such branching units and Reconfigurable Optical Add Drop Multiplexers (ROADMs) can at times return unexpected HLLB data points in the LMS data along with corresponding loopback delays. Faulty undersea fiber splices may also create similar loopback reflections, as can bends in fiber caused by undersea earthquakes and other geological events. These extra peaks should be ignored by the LMS. However, if an unexpected peak is detected very close to an expected repeater peak, the wrong data point may be interpreted and an erroneous fault may be reported.